Pressure Stabilized Lithium-Aluminum Compounds with Both Superconducting and Superionic Behaviors

Superconducting and superionic behaviors have physically intriguing dynamic properties of electrons and ions, respectively, both of which are conceptually important and have great potential for practical applications. Whether these two phenomena can appear in the same system is an interesting and important question. Here, using crystal structure predictions and first-principle calculations combined with machine learning, we identify several stable Li-Al compounds with electride behavior under high pressure, and we find that the electronic density of states of some of the compounds has characteristics of the two-dimensional electron gas. Among them, we estimate that ${\mathrm{Li}}_6\mathrm{Al}$ at 150 GPa has a superconducting transition temperature of around 29 K and enters a superionic state at a high temperature and wide pressure range. The diffusion in ${\mathrm{Li}}_6\mathrm{Al}$ is found to be affected by an electride and attributed to the atomic collective motion. Our results indicate that alkali metal alloys can be effective platforms to study the abundant physical properties and their manipulation with pressure and temperature.

Figure 1

Energetic stability of the Li-Al system under pressure and structural features of some interesting, stable, high-pressure Li-Al compounds. (a) Calculated convex hulls for the Li-Al compounds at different pressures (50, 100, and 150 GPa). Solid and open symbols represent the thermodynamically stable and metastable compounds. (b) Pressure-composition phase diagram of the Li-Al compounds. (c) Crystal structure of $C2/m$ ${\mathrm{Li}}_6\mathrm{Al}$ at 100 GPa. (d,e) Layered feature of the $C2/m$ ${\mathrm{Li}}_6\mathrm{Al}$ phase and the $Cmcm$ ${\mathrm{Li}}_3\mathrm{Al}$ phase. (f) Crystal structure of $I4/mmm$ ${\mathrm{Li}}_2\mathrm{Al}$ at 50 GPa. In these structures, magenta and cyan balls correspond to Al and Li atoms, respectively.

Figure 2

Electronic properties and superconducting properties of $I4/mmm$ ${\mathrm{Li}}_2\mathrm{Al}$ and $C2/m$ ${\mathrm{Li}}_6\mathrm{Al}$. (a) Band structures and PDOS of $I4/mmm$ ${\mathrm{Li}}_2\mathrm{Al}$ at 50 GPa. We show electron density maps of the bottom of the valence band ($E=-10$ to $-7.5\mathrm{eV}$, isosurface of 0.005), which gives the 2D electron gas features in real space. (b) ELF on the $(11-1)$ plane and partial electron density map within an energy ($=50\mathrm{meV}$) corresponding to near the Fermi level of $C2/m$ ${\mathrm{Li}}_6\mathrm{Al}$ at 100 GPa. (c) Phonon band structures, projected density of states, Eliashberg spectral function, and electron-phonon coupling constant $\lambda$ of ${\mathrm{Li}}_6\mathrm{Al}$ at 150 GPa. The sizes of the solid violet circles are proportional to the electron-phonon coupling strength.

Figure 3

Finite-temperature thermodynamic behaviors of ${\mathrm{Li}}_6\mathrm{Al}$ at different temperatures. (a)–(c), Snapshots extracted from MLMDs at (a) 800 K, (b) 1600 K, and (c) 2000 K, which clearly show the behaviors of solid, superionic, and liquid states. (d) Volume-temperature relationship of ${\mathrm{Li}}_6\mathrm{Al}$ at 150 GPa. The discontinuity in volume vs temperature confirms the melting. Diffusion coefficients at different $T$ are displayed via Arrhenius linear regression. Dashed lines are guides to the eye, and green horizontal lines in property symbols are error bars. (e) Radial distribution functions $g(r)$ in solid, superionic, and liquid states. Black vertical lines highlight Li-Li structural short- and long-order changes. (f) Simplest atomic collective diffusion (ring mechanism) with their ELF (0.75) environments, indicating that the superionic state in Li-Al compounds is connected with their electride feature. Four red lithium atoms rotate almost simultaneously above an Al atom that is surrounded by localized electrons.

Figure 4

Proposed temperature-pressure phase diagram of the $C2/m$ ${\mathrm{Li}}_6\mathrm{Al}$. The symbols represent the data points corresponding to four distinct states: triangle, solid state; square, superionic state; circle, liquid state; and diamond, superconducting state. The black dashed lines are the phase boundaries.